End enclosure for transformers

ABSTRACT

An end enclosure for transformers in which a pair of opposed transverse walls are formed along bend lines which form a square corner with the top wall, said sidewalls having elongated openings which extend up to said bend lines to provide greater access space for conductors. The opposite pair of transverse walls may have top round corners so that said walls tend to counter displacement towards each other. Interlock means may be provided between the first and second pair of sidewalls to prevent outward displacement of the round corner sidewalls and inward displacement of the square corner sidewalls. A method for making such improved end enclosures from planar, deformable sheet material provides removing areas of selected configuration from the four corners of the sheet material, and forming the opposed pair of extending sidewall portions into the sidewalls of the enclosures by moving them along bend lines. A substantially continuous flange portion is formed by moving a bottom portion of each sidewall along bend lines.

United States Patent [191 Franz et a1.

END ENCLOSURE FOR TRANSFORMERS [75] Inventors: Robert W. Franz, Elk Grove Village; John J. Zielnicki, Northbrook, both of Ill. [73] Assignee: Cam-Fram Tool Co., Inc., Chicago,

Ill.

[22] Filed: Jan. 15, 1973 [211 App]. No.: 323,728

Related [1.8. Application Data [62] Division of Ser. No. 215,058, Jan. 3, 1972, Pat. No.

52 us. C1. 336/98 [51] Int. Cl. H0lt 27/02 [58] Field of Search 336/90, 98, 210; 174/DIG. 2; 113/120 UE [56] References Cited UNITED STATES PATENTS 3,219,884 ll/l965 Chessin et al l74/DIG. 2 2,126,174 8/1938 Comstock 336/210 X 2,741,528 4/1956 Clark et al..... 336/210 X 6/1941 Miller.....' 336/98 X Primary Examiner-Thomas J. Kozma 5 7 ABSTRACT An end enclosure for transformers in which a pair of opposed transverse walls are formed along bend lines which form a square corner with the top wall, said sidewalls having elongated openings which extend up to said bend lines to provide greater access space for conductors. The opposite pair of transverse walls may have top round corners so that said walls tend to counter displacement towards each other. Interlock means may be provided between the first and second pair of sidewalls to prevent outward displacement of the round corner sidewalls and inward displacement of the square corner sidewalls. A method for making such improved end enclosures from planar, deformable sheet material provides removing areas of selected configuration from the four comers of the sheet material, and'forming the opposed pair of extending sidewall portions into the sidewalls of the enclosures by moving them along bend lines. A substantially continuous flange portion isformed by moving a bottom portion of each sidewall along bend lines.

10 Claims, 15 Drawing Figures PATENTEDMAY 1 I974 SHEET 3 0F 3 E wE END ENCLOSURE FOR TRANSFORMERS This application is a division of application Ser. No. 215,058, now US. Pat. No. 3,762,205, filed Jan. 3, 1972.

This invention relates to new end enclosures for transformers, and to a method for making such end enclosures. The invention particularly relates to an improved end enclosure which can be provided with one or more elongated openings that extend up to the junction or bend line between the sidewall and top wall enclosure. The invention further relates to a highly efficient and economical method for making such improved enclosures including a continuous, automated method wherein such enclosures are formed along an advancing strip of deformable material.

End enclosures for transformers are now commonly made by a draw die wherein a sheet of deformable material forms a unitary structure having a top wall, sidewalls, and a substantially continuous flange at the bottoms of the sidewalls. Such enclosures are formed by a die element which deforms the sheet material around an anvil to obtain an enclosure with uninterrupted top walls, sidewalls and base flange.

Elongated openings are then punched out of the sidewalls of such enclosures by die punches which, however, must clear the anvil element inside the enclosure. Such an anvil element understandably. takes up space below the top wall, thereby preventing upper portions of the sidewalls from being perforated by the punch die. As a result, the elongated openings do not extend up to the junction line with the top wall. Such limited height of the elongated openings provides a smaller space through which conductors are down fromthe transformer for connections with other electrical components. The art has recognized that such limited heights of the openings results in often tightly bending the conductors, or constraining them within small spaces prior to drawing them through the openings. This has led to the very serious problem of damage to the insulator coating of the conductor, thereby causing shorts and fires. The known techniques in the art for making such end enclosures for transformers are inadequate to solve the problem oflimited height ofthe elongated openings or slots for the conductors. So long as the anvil element must be positioned within the enclosure in the deformation and punching step, a certain upper portion area of each sidewall cannot be available for formation of the elongated opening. Such elongated openings customarily have an upward closed end which must be formed belowthe die element, that is, substantially below the top wall or the junction of the sidewall with the top wall.

The problem of limited height of the elongated opening occurs no matter which sidewall of the enclosure is selected. Such elongated openings are customarily.

formed on each of the four sidewalls, commonly two openings in each sidewall. Every opening must, of necessity, have its closed end positioned substantially below the top planar wall to thereby limit the available space of the opening.

The prior art unitary enclosures have the disadvantage of shortened openings, and have other disadvantages associated with their otherwise desirably rigid structure. The unitary structure results in a lack of versatility in the formation process because such an enclosure can be formed essentially only by a draw die process over an anvil element. This seriously limits or eliminates the introduction of economical steps or innovations in the process which would tend to lead to the production of end enclosures in a more rapid and economical way.

It is accordingly one important object of the present invention to provide an improved end enclosure for transformers, and a method for making same, wherein sidewalls of the enclosure can have one or more elongated openings for conductors, which height is not limited in the sidewall areas. In this way, an end enclosure of given height can lead to elongated openings providing more available space through which conductors can be passed with consequent reduction in constraining movements of such conductors.

Another important object of the present invention is to provide an improved end enclosure, and method for making same, in which a rugged, operable end enclosure has separable sidewall portions which are moved along bend lines to form sidewalls lying in planes substantially normal to the top wall portion, at least some of such bend lines being square cornered.

Another important'object of the present invention is an improved end enclosure for a transformer, and a method of making same, wherein an operable end enclosure is formed from a planer sheet of deformable material in which separate sidewall portions extend in coplaner fashion from a top wall portion, and wherein elongated slot openings are formed in the sidewall portions while the sheet material is in planer disposition so that the closed ends of such elongated openings can be formed higher in sidewall areas.

.Yet another important object of the present invention is an improved end enclosure, and method for making same, wherein one pair of separated sidewall portions can be formed with square corners, and another pair of sidewall portions can be formed with bend lines following a radius to impart greater rigidity against displacement of said radiused sidewalls.

Yet still another important object of the present invention is an end enclosure for transformers, and a method for making same, wherein separated sidewall portions can be efficiently moved by forming elements into sidewall positions in planes substantially normal to the top wall portion, and wherein such moved sidewall portions can be efficiently interlocked to counter tendencies against inward displacement of one pair of sidewalls and outward displacement of the other pair of sidewalls.

Still yet another important object of the present invention is an improved method for making end enclosures for transformers wherein strips of deformable sheet material are advanced through progressive die stations so that sidewall portions, flange portions, and elongated openings may be speedily formed to obtain the completed, assembled and interlocked end enclosure following discharge from a last cutoff station.

Still yet another important object of the-present invention is to provide an improved method for making end enclosures for transformers wherein a continuous advancing strip of deformable sheet material can be advanced through progressive die stations to bilaterally form enclosures on each side of a central carrier strip portion so that a single strip can be used to increase end enclosure production.

Yet still another important object of the present invention is to provide an improved method for making end enclosures for transformers wherein a plurality of elongated openings can be punched out on each of four sidewall portions while such sidewall portions are disposed in coplaner fashion with a central top wall portion. Some of said openings have closed ends at about square corner bend lines and others have closed ends within the radius of bend lines, all of the closed ends of said elongated openings being formed without restricting their placements as a result of the forming elements used in the process.

The foregoing-objects are now attained by the present invention shown in the following disclosure. Still other objects may be attained as they occur to practitioners from time to time upon considering such disclosure which includes drawings, wherein:

FIG. 1 is a perspective view showing the improved end enclosure of the present invention mounted in a transformer;

FIG. 2 is a perspective view of the end enclosure, on a reduced scale;

FIG. 3 is a perspective view showing the inside of the end'enclosure of FIG. 2;

FIG. 4 is a sectional view of the end enclosure, on a reduced scale, taken along a plane following the short axis of the end enclosure shown in FIG. 3;

FIG. 5 is a plan view of a planer deformable sheet material blank used to form the end enclosure shown in the foregoing views;

FIG. 6 isa schematic illustration, with parts and portions removed, showing a strip of deformable material movable through progressive die stations leading to the formation of the end enclosure;

FIG. 7 is a view along line 7-7 of FIG. 6, partly in section and with elements added for purposes of clary;

FIG. 8 is a view along line 88 of FIG. 6, further illustrated in the same manner as FIG. 7;

FIG. 9 is a view along line 99 of FIG. 6, further illustrated in the same manner as FIG. 7;

FIG. 6A is a schematic .illustration similar to the view of FIG. 6, but showing additional progressive die stations which complete the formation of the end enclosure;

FIG. 10 is a view along line 10- 10 of FIG. GA, on an enlarged scale, partly in section, and with elements added for purposes of clarity;

FIG. 11 is a view along line ll-l1 of FIG. 6A, and further shown in the same manner as the view of FIG. 10;

FIG. 12 is a view along line 12-l2 of FIG. 6A, and further shown in the same manner as the view of FIG. 0;

FIG. 13 is a view along line l3l3 of FIG. 6A, and further shown in the same manner as the view of FIG. 10; and

FIG. 14 is a view along line 14--l4 of FIG. 6A, and further shown in the same manner as the view of FIG. 10.

The use of the same numerals in the different views of the drawings will indicate a reference to the same structures, parts or elements, as the case may be.

The improved end enclosure shown generally as 16 is seen mounted in a transformer 18 in the view of FIG. 1. The operable end enclosure separated from the transformer is shown in following FIGS. 2-4.

The end enclosure shown as having a flat top wall 20. A pair of opposed square corner sidewalls 22 are joined to the top wall along a bend line or square corner 24.

Another pair of opposed round corner sidewalls 26 are joined to'this top wall by a round corner or bend line 28 which is formed along a radius.

Each sidewall is shown with an adjoining flange portion, such as square corner sidewall flanges 30, which are formed along a bend line 31 so that such flanges are substantially perpendicular to the plane of the adjoining sidewall. The round corner sidewalls 26 also have a round corner sidewall flange similarly disposed along abend line 33.

A plurality of elongated openings 34 are shown in each sidewall. Through such openings, conductors 35 pass outside the transformer for connection to various electrical components. Each elongated opening is shown as having an upper, closed top 36, and an opposite, lower open bottom 38. The elongated opening is provided with an outwardly bent lip 40 which is customarily provided to keep the edges of the opening out of damaging contact with the insulation cover of the conductor 35. It will be seen that the plurality of elongated openings 34 in the square corner sidewalls 22 have their closed ends right up to square cornersor bend lines 24. It will further be seen that the closed ends of elongated openings 34 in the round corner sidewalls 26 are positioned within the formed corner radius or bend line 28. The open ends 38 of the respective elongated openings are shown positioned in the various flange portions adjoining the respective sidewalls.

The opposite ends of the square corner sidewalls 22 are shown with locking tab extensions 42. In the assembled enclosure, the tabs'are bent to overlap the opposite ends of the round corner sidewalls 26, as shown. Such locking tabs 42 form a part of an efficient interlock between the respective sidewalls as will be shortly described in greater detail. The overlapping extension tabs 42 retain roundcornered sidewalls 26 from being displaced outwardly or away from each other. The round corner 28 also impartsadded rigidity to counter any tendencies of round corner sidewalls 26 from being displaced towards or away from each other.

The end enclosure shown in the foregoing views is formed from the cutout blank sheet of deformable material shown in the view of FIG. 5. Such a planer blank has a pair of opposite square corner sidewall portions 22, and another pair of opposite round corner sidewall portions 26, both extending in normal relationship to central top wall portion 20. The square corner sidewall portions 22 are moved along square corner bend line 24 so that they lie in planes which are normal to the plane of the top wall portion 20. The round corner sidewall portions 26 are moved downwardly to form curved bend line-28, such round cornered sidewalls also lying in aplane which is nornial to the plane of the top wall portion 20.

In the illustrated view of FIG. 5, flange portions 30, at the bottom of square corner sidewall portion 22, are bent outwardly along bend lines 31 so that such flange portions are substantially perpendicular to the sidewall portions. In the same way, flange portions 32 on the round corner sidewalls are moved along bend lines 33 to be similarly fashioned. It is seen that flange portions 32 along sidewall portions, 26 extend beyond the opposite ends of the round corner sidewalls a distance equivalent to the thickness of flange portions 30. As a result, the assembled end enclosure forms a substantially continuous flange portion at the bottom of the respective sidewalls, interrupted only by the open ends 38 of the elongated openings 34.

The interlock in the formed end enclosure is enhanced by features in addition to locking tabs 42. The round corner sidewalls 26 have opposite ends 44 which are inboard of shoulders 45. The straight corner sidewalls 22 have upper opposite arcuate corners 46 which are inboard of lower opposite straight corners 47. In assembled form, this inside-outside position prevents inward displacement of the respective sidewalls, whereas the locking tabs prevent outward displacement of the round corner sidewalls. The frictional contact of the locking tabs 42 against the round corner sidewalls also helps to prevent outward displacement of square corner sidewalls 22, that is, away from each other.

It will be apparent that the blank cutout shown in FIG. 5 can be variously cut out by removing areas of selected configurations from four corners of a sheet of deformable material such as rolled steel. Following the removal of such configurations, the resulting blank shown in FIG. 5 can be assembled by moving the square corner sidewalls downwardly to the desired position, then moving the round corner sidewalls downwardly to the desired position where the opposite ends adjoin the opposite ends of the square corner sidewalls. The upper arcuate corner 66 are then positioned inside the shoulders 45, and the lower most straight corners 47 are positioned outside the ends 44. The tab extensions 42 are then bent towards each other to overlap the end 44 of the round corner sidewalls. Flange portions 30 and 32 are then bent outwardly until they are substantially perpendicular to'the planes of their adjoining sidewalls.

Theimproved end enclosure is likewise adapted for rapid automated production by a continuous method such as that shown in following views of FIGS..614. Looking first at FIGS. 6 and 6A, various progressive die stations are indicated by Roman numeral designations. A strip of deformable sheet material is continually advanced in a direction indicated to the right of the views of FIGS. 6 and 6A. The illustrated strip is used to make two enclosures simultaneously, one on each side of a center carrier strip indicated at 50. One lateral strip formation-is indicated at A, and this strip formation shall be referred to for purposes of a detailed description, although .it should be understood that the same structures, elements and parts occur simultaneously on a strip formation partly shown and indicated at B. The entire strip may therefore be considered as being bilaterally symmetrical, that is, along the longitudinal axis of common carrier strip 50.

A blank cutout station is generally at I. The areas of selected configuration are removed at this station from the blank sheet material to form a blank of extending sidewall portions. A slotting station is indicated II where elongated slots are formed in the respective sidewall portions. A first lip forming station is shown at III, and this initially forms a lip around the elongated openings in the sidewall portion. A lancing station is shown at IV wherein strip portions are separated so that the transverse wall portions may be moved into position to form the end enclosure. The lip formation is completed at station V where the curve is desirably flared, and where an interlocking element is also moved into position. A partial formation of the sidewalls is indicated at the partial form station VI. A final form round corner sidewall station is indicated at VII, and an inside forming station indicated at VIII for forming a round corner sidewall adjacent the common carrier strip 50. The square corner sidewalls are completed at a final form station indicated by IX. The finally formed end enclosure is then separated from the carrier strip 50 at cutoff station X. j

The sheet of deformable material is advanced between an upper die component collectively indicated at 52 and a lower die component collectively indicated at 54. Some of these components may be more particularly identified, but they generally shall represent the type of components operating in progressive die assemblies. Such components are variously actuated by conventional means which will not be described. The structural features of the die components will likewise not be particularly described, since the practitioners will readily understand such features from considering the formations at the various stations. The strip of advancing material shown in FIGS. 6 and 6A are illustrated without the upper and lower die components, but it is understood that such components are present as indicated somewhat in the partly sectioned views.

In the blank cutout station notched area 56 and cutout area 58 of selected configuration are formed by removing the pieces. A forming element shown as a cutout punch 59 removes a knockout piece 60. An appropriately formed die element which is not shown will likewise knockout a piece to form the cutout area 58.

The advancing strip is selectively positioned at the respective stations by means such as pilot location holes 61 which are engaged by pilot tubes, portionally indicated at 62. It is seen that repeated removal of areas 56, 58 results in formation of sidewall portions extending from an extending topwall portion. It is fur ther seen that the illustrated embodiment, round corner sidewall portions 26 extend in a direction which is normal to the direction of travel of the advancing strip. Cutout areas have formed the flange portion 32 which is freely positioned at the bottom of one of the round corner sidewalls 26. The opposed roundcorner sidewall is joined to the common carrier strip 50, and such wall is formed, except for the flange portion, by cutout areas 58. i

' The cutout areas 56 and 58 also form web portions 63 which have two, undivided square corner sidewall portions. The undivided square corner sidewall portion are in mutual mirror orientation with respect to their adjoining bottom edges which are not, however, separated at this station. The web portion 63 advances in a direction coincidental with the direction of the advancing strip.

Slotting station II forms the elongated openings in the respective sidewall. Cutouts 64 are seen in the round corner sidewall, and cutouts 65 are seen in the square corner sidewalls. The cutouts in the round corner and square corner sidewalls are formed by punch elements 66,67. It will be seen that cutouts 64 are enlarged to a double cutout configuration 68 when the web portion 42 advances through theslotting station. Punch elements similar to 66 or 67 are used to punch the cutouts in reverse relationship to the cutout 64 so the resulting double cutout 68 is formed as shown. When such web portion is separated along a generally central line, the double cutout 68 will be symmetrically bisected to form elongated openings in each of the two separated round corner sidewalls. It will be seen that the elongated openings have a reduced width portion 69, such as on cutout 64, 65; and such as is present at both opposite ends in cutouts 68. This reduced portion will be later opened up to substantially the same width as the larger cutout dimensions following a curling step wherein the rolled back sheet material will be in the form of a lip 40.

Station lll illustrates the curling operation. A curl punch element 70 operates as a lower component to initially form a substantially straight curl 71 extending from the plane of the web portion 63. A similar punch element 73 forms a like straight curl 71 in the paired elongated openings 65 and 69. At the following station IV, the double round corner sidewalls in the web portion 63 are spearated by a lancing or cutting step. The lance die component 75 cuts the web. portion so that the round corner sidewalls separate along separation line 74.

In the following station V, the straight curls are completely formed by punch element 78 which turns the straight curl over at rounded corners to flare or to form the lip 40 as it finally appears in the assembled end enclosure. At the same station, the locking tabs 42 are bent back so they are in a plane which is normal to the square corner sidewalls and parallel to the round corner sidewalls. Tab bending punches 80 with right angle faces are used to turn the locking tabs into said bent position.

Following station Vl is a partial sidewall forming station in which a flange and sidewall punch 82 with a right angle dihedral face 84 forms the flange portions 32, and partially bends both square corner sidewalls and the free round corner sidewall. The round corner sidewall fixed to the common carrier strip 50 is formed at a later station.

One of the round corner sidewalls is formed at station Vll by the round corner sidewall punch 86- having an arcuate forming face. Such punch completes the movement of the sidewall from the partially turn position to the finally moved and formed position as indicated in the view of H0. 11.

The opposite round corner sidewall is formed at following station Vlll by inside sidewall punch 88, which has an outside corner radius that cooperatively acts with corresponding inside corner radius of the upper component 52 to form the round corner of the sidewall.

. carrier strip 50 to simultaneously form flanges 32 and to drop the completed end enclosures into a receptaclw, not show.

The claims of the invention are now presented. What is claimedis: 1. An end enclosure for a transformer, including a body having opposite pairs of sidewalls, a top wall, and a substantially continuous flange at the bottom of the sidewalls,

one pair of opposite sidewalls forming square corners with the top wall along a bend line,

the opposite ends of said square corner sidewalls adjoining the respective opposite ends of the other pair of opposite sidewalls,

each of the sidewalls having a flange portion bent outwardly and extending to opposite ends of such sidewalls,

said individual flange portions defining said substantially continuous flange at the bottom of the respective sidewalls, and

at least one elongated opening in a square corner sidewall, said opening having an upper closed end substantially at the bend line of the square corner sidewall, and an opposite, lower open end in the flange portion of said square corner sidewall.

2. An end enclosure for a transformer as in claim 1, wherein said opposite pair of sidewalls are joined to said top wall by a corner formed on a radius to define round corner sidewalls. I

3. An end enclosure for a transformer as in claim 1, wherein an interlock is formed between adjoining opposite ends of'the respective sidewalls.

4. An end enclosure for transformer as in claim 3, wherein said interlockis formed at least in part, by tab extensions at opposite ends of each square corner sidewall, said tab extension being bent to overlap the opposite ends of said opposite pair of sidewalls.

5. An end enclosure for a transformer as in claim 2,

wherein the opposite ends of each square corner side wall have a locking tab bent to overlap the opposite ends of each round corner sidewall.

6. An end'enclosure for transformer as in claim 5', wherein the opposite upper corners of each straight corner wall are arcuate, and wherein the radiused corners of each round corner sidewall follow such arcuate corners.

7. An end enclosure for a transformer as in claim 6, wherein said arcuate corners are inside the radius of the round corner sidewalls, and wherein the lower opposite corners of the straight cornered sidewalls are straight and extend beyond said arcuate corners, said lower opposite straight corners being outside the opposite ends of the round corner sidewalls.

8. An end enclosure for a transformer as in claim 7, square corner sidewall has a plurality of like elongated openings, each of said openings having an upper closed end substantially at the bend line of the square corner sidewall and an opposite lower open end in the flange portion of square corner sidewall, and each of said elongated openings having an outwardly formed lip at said closed end. I

9. An end enclosure for a transformer as in claim 8, wherein each of said round corner sidewalls has at least one elongated opening, said elongated opening having an upper closed end within the radius of the round..cor-' ner sidewall, an opposite, lower open end in the flange portion of said round corner sidewall, and an outwardly formed lip at said closed end.

l0. Anend enclosure for a transformer as in claim 9, where each round corner has a plurality of like elongated openings, each of said openings having an upper closed end within the radius of the round corner sidewall, a lower open end in the flange portion of the round corner sidewall, and each opening having an outwardly formed lip at theclosed end. 

1. An end enclosure for a transformer, including a body having opposite pairs of sidewalls, a top wall, and a substantially continuous flange at the bottom of the sidewalls, one pair of opposite sidewalls forming square corners with the top wall along a bend line, the opposite ends of said square corner sidewalls adjoining the respective opposite ends of the other pair of opposite sidewalls, each of the sidewalls having a flange portion bent outwardly and extending to opposite ends of such sidewalls, said individual flange portions defining said substantially continuous flange at the bottom of the respective sidewalls, and at least one elongated opening in a square corner sidewall, said opening having an upper closed end substantially at the bend line of the square corner sidewall, and an opposite, lower open end in the flange portion of said square corner sidewall.
 2. An end enclosure for a transformer as in claim 1, wherein said opposite pair of sidewalls are joined to said top wall by a corner formed on a radius to define round corner sidewalls.
 3. An end enclosure for a transformer as in claim 1, wherein an interlock is formed between adjoining opposite ends of the respective sidewalls.
 4. An end enclosure for transformer as in claim 3, wherein said interlock is formed at least in part, by tab extensions at opposite ends of each square corner sidewall, said tab extension being bent to overlap the opposite ends of said opposite pair of sidewalls.
 5. An end enclosure for a transformer as in claim 2, wherein the opposite ends of each square corner sidewall have a locking tab bent to overlap the opposite ends of each round corner sidewall.
 6. An end enclosure for transformer as in claim 5, wherein the opposite upper corners of each straight corner wall are arcuate, and wherein the radiused corners of each round corner sidewall follow such arcuate corners.
 7. An end enclosure for a transformer as in claim 6, wherein said arcuate corners are inside the radius of the round corner sidewalls, and wherein the lower opposite corners of the straight cornered sidewalls are straight and extend beyond said arcuate corners, said lower opposite straight corners being outside the opposite ends of the round corner sidewalls.
 8. An end enclosure for a transformer as in claim 7, square corner sidewall has a plurality of like elongated openings, each of said openings having an upper closed end substantially at the bend line of the square corner sidewall and an opposite lower open end in the flange portion of square corner sidewall, and each of said elongated openings having an outwardly formed lip at said closed end.
 9. An end enclosure for a transformer as in claim 8, wherein each of said round corner sidewalls has at least one elongated opening, said elongated opening having an upper closed end within the radiUs of the round corner sidewall, an opposite, lower open end in the flange portion of said round corner sidewall, and an outwardly formed lip at said closed end.
 10. An end enclosure for a transformer as in claim 9, where each round corner has a plurality of like elongated openings, each of said openings having an upper closed end within the radius of the round corner sidewall, a lower open end in the flange portion of the round corner sidewall, and each opening having an outwardly formed lip at the closed end. 